1. Field of the Invention
This invention relates to automatic gain control and more particularly to automatic gain control in hybrid analog and digital camera and imaging systems.
2. Description of Related Art
Conventionally, gain control in camera and imager devices and systems is subject to considerable technical limitations which make it difficult to use current cameras for professional and consumer applications. In particular, gain control is undertaken with an analog system or with a digital system, lacking integrated controls which are effective for both the analog as well as the digital portions of a combined camera system. There is a need to separately control gain over the analog and digital subsystems.
According to the present invention, a signal processing system (SPS) for an imager device having a settable shutter includes a digital gain circuit, an analog gain circuit, a shutter gain circuit, and an automatic gain control (AGC) circuit, for controlling the digital, analog, and shutter gain blocks. According to the present invention, gain control of a signal processing system for an imager device includes an automatic gain control (AGC) circuit for controlling multimode levels of gain provided to selected gain modules of the camera system. According to one embodiment of the present invention, an automatic gain control (AGC) circuit includes a gain splitter circuit for receiving gain values which have been determined. The gain splitter circuit produces distributed gain values from the received gain values. The distributed gain values include shutter gain values for an imager device or CCD camera, as well as analog or VGA gain values, and digital gain values. According to another embodiment of the present invention, the AGC circuit provides at least a minimum level of chip gain, which is settable, and which is then divided into analog or VGA gain values and digital gain values. According to another embodiment of the present invention, the AGC circuit provides selectable threshold multimode gain control according to which chip gain values are provided between a user-selectable minimum gain value and a maximum level which is user-selectable. According to the present invention, the total gain is continuous across the thresholds of minimum shutter gain and minimum chip gain. Gain change according to the present invention is incremental according to predetermined gain step sizes. As total gain is increased beyond the user-established maximum shutter gain value, the incremental change in gain is performed in substantially the same gain step sizes. Accordingly, subsequent gain increases beyond the shutter maximum gain is accomplished with another gain mode, i.e., the analog or VGA gain mode, and remains user-transparent or invisible. Similarly according to the present invention, as total gain is increased beyond the maximum analog or VGA gain value, the incremental change in gain is performed in substantially the same gain step sizes. Accordingly, subsequent gain increases beyond the analog or VGA maximum gain are accomplished with another gain mode, i.e., the digital gain, and remains user-transparent or invisible. Shutter gain according to the present invention accordingly can be increased monotonically from zero to a user-settable shutter gain level threshold. The total maximum gain is additionally user-settable, and if the current automatic gain level exceeds the shutter gain maximum, the excess thereover is provided as chip gain, beyond any already established or set minimum chip gain level. Similarly, if the current automatic gain level exceeds the sum of the shutter gain maximum and the analog or VGA gain maximum, then additional gain is provided as digital chip gain up to a user-settable total maximum gain. According to the present invention, total chip gain is increasable substantially continuously without breach, in predetermined level steps across mode thresholds at which gain continues to progress, beyond which one or more particularized gain types cannot increase.